Picture in your mind two dogs meeting and greeting, off-leash. They engage in a social dance – advancing, retreating, moving around each other, sniffing various body parts, giving body language signals intended to keep the interaction civil. Sometimes the movements are slow; sometimes they are quick. If one dog is cautious or fearful of the other, he can retreat as he wishes, using social distance to keep himself safe.

Now picture those same two dogs meeting on-leash. The dance is stilted, inhibited by the restraint of the leash. One dog tries to circle the other, and the leash tangles around his legs. The cautious dog would like to retreat to safety but knows the leash restricts his movement, and elects to act out his second option to increase distance – a growl and a snap to signal to the other dog to move away – who cannot because he is leashed. The fight is on.

In the future, the cautious dog will offer a growl and snap before he’s close enough for the other dog to make contact. The best defense is a good offense. Alarmed, owners move away from each other, and the fearful dog’s aggression is reinforced by the increased distance. Behaviors that are reinforced repeat and increase and the cautious dog’s aggression escalates as he realizes that it’s a successful behavior strategy for him – it keeps other scary dogs away. You now have a leash-aggressive dog. Absent the leash, he still chooses to move away from the other dog – his first behavior choice.

Leash Restraint Frustration

This is the dog who would love to visit with other dogs, on- or off-leash. In fact, you have reinforced his “I wanna go see the other dog!” behavior by frequently allowing him to greet other dogs on-leash, being careful to keep the leash loose, as your instructor showed you in class, to avoid having the leash interfere with the dogs’ normal social interaction. However, sometimes owners of approaching dogs give a clear indication that they don’t want to allow their dogs to greet yours. You politely comply, restraining your dog with his leash to hold him back.

Your dog, who suffers from low impulse control and low tolerance for frustration, becomes highly aroused when his desire to greet the other dogs is thwarted. His frustration and arousal turn into aggression, and your dog is labeled as “reactive.” Absent the leash, he would have happily greeted other dogs without any frustration or aggression.

When the Dog Owner is a Stressor

Stories abound of dogs who are ferocious if the owner is in the room, or holding the leash, yet turn into pussycats when the owner leaves or hands the leash off to someone else. One possibility is that the dog is protecting his owner. Another more likely one is that the owner’s presence gives a fearful dog enough confidence to act aggressively, but the dog shuts down when the owner leaves. Yet a third and perhaps most likely explanation is that the owner is stressed in anticipation of the dog’s behavior in the proximity of other dogs, as well as other coming events – such as obedience ring competition. The stress of the human is an added stressor for the dog who is well aware of his owner’s emotional distress (they are masters at reading human body language, remember?) and that’s enough to put him over his aggression threshold.

Prior Negative Association with Events that Happened On-Leash

If you ever made the mistake of punishing your dog for inappropriate behavior toward other dogs on-leash, you may have given him a negative association between the leash and the presence of other dogs. Verbal corrections, a jerk on the leash or, horrors, a zap from a shock collar, are all stressors that your dog might now associate with the presence of another dog when he’s on his leash. That added stress can cause him to be aggressive when he otherwise might not have been.

Perhaps he was attacked by another dog when he was on his leash. Attacks can create very strong negative associations; you will often see a dog who is on-leash-aggressive toward a particular breed, type, or color of a dog that reminds him of a prior aggressor.

Tightening of the Leash as a Stressor/Trigger

Sometimes introductions seem to be going well until one owner turns to leave, and pulls on the leash to get her dog to come along. Suddenly a fight breaks out for no apparent reason other than the leash tightening.

If things were already a bit tense between the dogs, the tightening of the leash can easily trigger an aggressive outburst. For one thing – it’s a stressor. Imagine if you were engaged in conversation with someone, and someone grabbed your arm – or worse, your shirt collar – and tried to pull you away. Annoying, yes? Well, annoying equals stress – perhaps enough stress to trigger an outburst.

For another, picture two dogs standing nose-to-nose, with some tension between them. Now envision the dog’s body language when his owner pulls back on the leash. The opposition reflex (scientifically named “thigmotaxis”) causes him to lean forward, tighten his muscles and stand a little taller – all of which mimic an assertive, perhaps aggressive body posture in a dog. If the annoyance-stress isn’t enough to trigger aggression in that dog, his offensive body language is likely to trigger an aggressive response from the other dog.

What to Do About Leash Aggression

Each of the causes for on-leash aggression has its own remedy. Some require preventative action – proactive steps that reduce the likelihood of future aggression. Others respond well to intervention in the actual moment of potential aggression. They all require that you keep a cool head when you identify possible trouble spots.

Leash interference with normal social interaction:

I make it a pretty strict rule not to introduce dogs while still holding leashes. My dog-to-dog introduction routine involves letting the dogs see each other from a distance and, if all appears copacetic as the dogs near each other, dropping leashes and letting the dogs interact normally, without human interference. If the meeting unravels, we can grab leashes and separate the dogs without risking a close encounter with flashing teeth. If all goes well, however, we remove leashes after a moment or two of greeting and allow the dogs to play freely.

Leash restraint frustration:

My rule about “no greetings on-leash” works well to proactively prevent leash-restraint frustration. Because my dogs aren’t in the habit of routinely greeting other dogs on-leash, they don’t develop that expectation, and therefore don’t become frustrated when it doesn’t happen. I teach my dogs that when they are on the leash, they are with me, and their job is to pay attention to me. Only when and if I drop the leash and give them the “Go play!” cue do they expect to interact with an approaching dog.

For this reason, we do not teach an on-leash greeting exercise in our good manners classes (greeting humans, yes; greeting dogs, no). We aim to convince our students of the value of the no-greeting rule from Day One of their first class, in part to decrease the potential for on-leash frustration-aggression.

Prior Negative Association with On-Leash Events:

The greater the intensity of the prior negative association, the harder you will have to work to overcome it.

If your dog has perceived that the presence of another dog causes pain or discomfort to his neck – from the shock of an electronic collar, the pinch of a prong collar, or even the choking of a flat collar – start by changing the equipment.

Then implement a counter-conditioning and desensitization program to other dogs. Dog at a distance makes chicken happen (the other dog needs to be far enough away (“sub-threshold”) so as not to elicit an aggressive response to your dog). Let him look at the other dog; feed him chicken. Let him look again; feed chicken. Look – feed. Look – feed. Over and over, until, when he sees another dog his brain thinks “Chicken!” – and he no longer stresses over the presence of the other dog.

Then gradually decrease the distance between your dog and the other dog, and increase intensity of the stimulus in other ways: multiple dogs, more movement from the dogs, direct eye contact with the other dogs, offset walking (walking in the same direction but not right next to each other); parallel walking (next to each other, dogs on the outside humans on the inside); offset approaches, and finally, direct approaches.

If your dog was actually attacked by another dog on one or more occasions, the negative association may be even more intense, and modification even more of a challenge.